Eccentric features in Saturn's outer C ring     

《Icarus》1987,72(2):437-467

A systematic search has been made for as yet unrecognized eccentric and inclined features in Saturn's outer C ring. The radii of all sharp-edged features in the outer C ring were measured in Voyager data consisting of six high-resolution images, the Photopolarimeter occultation data, and the Radio Science λ3.6-cm occultation data corrected for the effects of diffraction. Besides the well-known Maxwell ringlet at 87,491 km (1.450R_s), whose eccentric shape and kinematics have already been studied, two other narrow ringlets at 88,716 km (1.470R_S), and 90,171 km (1.495R_S) have been found to be demonstrably eccentric. The former has a mean width of ∼16 km and is located within a gap ∼30 km wide. The latter has a mean width of ∼62 km and is only partially isolated: its outer edge is defined by a gap ∼15 km wide. Though a coincidence of these two gaps with the Mimas 3:1 inner vertical and inner Lindblad resonances has been noted by previous workers, we find that neither ringlet shows conclusive evidence for the anticipated resonantly forced distortions. The 1.495R_S ringlet is best fitted by a model describing a freely precessing Keplerian ellipse with a radial amplitude of 2.8 ± 0.5 km. Neither a resonant forcing nor a free precession model fitted to the 1.470R_S ringlet provides conclusive results, though the latter is marginally better, yielding an amplitude no larger than ∼2.2 km. These two newly identified eccentric ringlets are compared with the previously studied Titan and Maxwell ringlets (C. Porco, P. D. Nicholson, N. Borderies, G. E. Danielson, P. Goldreich, J. B. Holberg, and A. L. Lane, Icarus 60 (1984), 1–16) and with the Uranian α, β, and ϵ ring.  相似文献   

Ringlets of the major planets which may consist of coagulated particles     

William D. Ross 《Earth, Moon, and Planets》1988,42(3):209-220

A ringlet of Saturn, Uranus, Neptune or Jupiter may be composed of particles held in contact by their mutual gravitation, without relative motion. Lacking tensile strength, each part of the ringlet orbits as if it were a separate particle, but all parts are constrained to the same orbit by their contacts. Slight shear strength prevents flow. This configuration is stable inside Roche's limit, and outside an inner limit within which it would scatter. These limits depend on the density of the ringlet. Conversely, for an observed radius in a ring, a range of possible density is calculated. For Saturn's ring system, the density of a ringlet at the inner edge of the C ring must be at least 2.0 g cm^-3 and in the outer F ring not more than 0.73. For Uranus, the inner ring must be at least 2.3, and the outer between 1.0 and 2.3. Jupiter's ring must be in the range 1.4 to 3.9, and Neptune's, in the range 0.6 to 1.5. In extended crowded regions of a ring system, the gaps between ringlets must be at least 38% as wide as the ringlets, in the outer portions of the system, and wider than that at smaller radii. Certain observations can be explained by this model, including the sharp edges of the rings, a long life of the system, the possible existence of a partial ring, asymmetry of brightness of Saturn ring A, and forward scattering of radio waves.  相似文献   

Morphology and variability of the Titan ringlet and Huygens ringlet edges     

Richard G. Jerousek  Larry W. Esposito 《Icarus》2011,216(1):280-291

We present a forward modeling approach for determining, in part, the ring particle spatial distribution in the vicinity of sharp ring or ringlet edges. Synthetic edge occultation profiles are computed based on a two-parameter particle spatial distribution model. One parameter, h, characterizes the vertical extent of the ring and the other, δ, characterizes the radial scale over which the ring optical depth transitions from the background ring value to zero. We compare our synthetic occultation profiles to high resolution stellar occultation light curves observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) for occultations by the Titan ringlet and Huygens ringlet edges.More than 100 stellar occultations of the Huygens ringlet and Titan ringlet edges were studied, comprising 343 independent occultation cuts of the edges of these two ringlets. In 237 of these profiles the measured light-curve was fit well with our two-parameter edge model. Of the remaining edge occultations, 69 contained structure that could only be fit with extremely large values of the ring-plane vertical thickness (h > 1 km) or by adopting a different model for the radial profile of the ring optical depth. An additional 37 could not be fit by our two-parameter model.Certain occultations at low ring-plane incidence angles as well as occultations nearly tangent to the ring edge allow the direct measurement of the radial scale over which the particle packing varies at the edge of the ringlet. In 24 occultations with these particular viewing geometries, we find a wide variation in the radial scale of the edge. We are able to constrain the vertical extent of the rings at the edge to less than ∼300 m in the 70% of the occultations with appropriate viewing geometry, however tighter constraints could not be placed on h due to the weaker sensitivity of the occultation profile to vertical thickness compared to its sensitivity to δ.Many occultations of a single edge could not be fit to a single value of δ, indicating large temporal or azimuthal variability, although the azimuthal variation in δ with respect to the longitudes of various moons in the system did not show any discernible pattern.  相似文献   

Theory of radio occultation by Saturn's rings     

Essam A. Marouf  G.Leonard Tyler  Von R. Eshleman 《Icarus》1982,49(2):161-193

The radio occultation technique is developed here as a new method for the study of the physical properties of planetary ring systems. Particular reference is made to geometrical and system characteristics of the Voyager dual-wavelength (13 and 3.6 cm) experiment at Saturn. The rings are studied based on the perturbations they introduce in the spectrum of coherent sinusoidal radio signals transmitted through the rings from a spacecraft in the vicinity of the planet to Earth. Two separate signal components are identified in a perturbed spectrum: a sinusoidal component that remains coherent with the incident signal but is reduced in intensity and possibly changed in phase, and a Doppler-broadened incoherent component whose spectral shape and strength are determined by the occultation geometry and the radial variation of the near-forward radar cross section of illuminated ringlets. Both components are derived in terms of the physical ring properties starting from a conventional radar formulation of the problem of single scattering on ensembles of discrete scatterers, which is then generalized to include near-forward multiple scattering. The latter is accomplished through special solutions of the equation of transfer for particles that are larger than the wavelength. When the occultation geometry is optimized, contributions of an individual ringlet to a perturbed spectrum can be identified with radial resolution on the order of a few kilometers for the coherent component and a few hundred kilometers for the incoherent one, thus permitting high-resolution reconstruction of the radial profile of the optical depth, as well as reconstruction of the radar cross section of resolved ringlets. Simultaneous estimates of the optical depth and radar cross section of a ringlet at 3.6 cm-gl allow separation of its aerial density and particle size, if the particles are of known material and form a narrow size distibution with radii greater than several tens of centimeters. This separation is also achieved for radii ?10 cm from differential effects on the coherent signal parameters at 3.6- and 13-cm wavelengths. For the more general case of a broad size distribution modeled by a power law, the absence of differential effects on the coherent signal binds the minimum size to be ?10 cm. In this case, the radius inferred from an estimate of the radar cross section represents an equivalent radius, which is strongly controlled by the maximum size of the distribution provided that the power index is in the range 3 to 4. On the other hand, detection of differential coherent signal extinction determines an upper bound on the maximum size and a lower bound on the power index, assuming water-ice particles. These bounds, together with an inferred equivalent size, constrain the size distribution at both its small and large ends.  相似文献   

Inelastic collisions in narrow planetary rings     

《Icarus》1987,71(1):78-90

It is shown that a particle ring with energy dissipation has an extremum in its energy when all the particles are in the same circular orbit. This extremum is a relative maximum in radial directions, indicating possible radial expansion; but it is a relative minimum in the particle velocity components, indicating a tendency for the velocity distribution to collapse. An N-body model of ring evolution incorporating two-body dynamics, oblateness perturbations, inelastic collisions, and phase averaging is described. By local analysis of impact statistics, it is shown that the velocity distribution of the ring will collapse if the coefficient of restitution ∈ ≲ 0.7. The collapse of the velocity distribution stabilizes a ring of point particles against radial dispersion. Furthermore, for ∈ ≲ 0.25, the semimajor axis distribution tends to collapse toward its local mean value, leading to radial collapse. A survey of ring evolution is presented for different values of coefficient of restitution and initial velocity dispersion. As has been predicted uy Goldreich and Tremaine, rings with equilibrium velocity distributions are unstable and expand, while rings where the velocity distribution collapses are shown to undergo massive, pervasive fragmentation into a myriad of ringlets. It is proposed that such fragmented rings are stable in their own right, and that the observational test to discriminate between the two cases is simply the presence of a smooth, featureless surface or the presence of intricate radial structure in the ring.  相似文献   

The microwave opacity of Saturn's rings at wavelengths of 3.6 and 13 cm from Voyager 1 radio occultation     

G.Leonard Tyler  Essam A. Marouf  Richard A. Simpson  Howard A. Zebker  Von R. Eshleman 《Icarus》1983,54(2):160-188

Radio occultation observations of Saturn's rings with Voyager 1 provided independent measurements of complex (amplitude and phase) microwave extinction and near-forward scattering cross section of the rings at wavelengths (λ) of 3.6 and 13 cm. The ring opening was 5.9°. The normal microwave opacities, τ[3.6] and τ[13], provide a measure of the total cross-sectional area of particles larger than about 1 and 4 cm radius, respectively. Ring C exhibits gently undulating (～ 1000 km) structure of normal opacity τ[3.6] ? 0.25 except for several narrow imbedded ringlets of less than about 100 km width and τ[3.6] ～ 0.5 to 1.0. The normalized differential opacity Δτ/τ[3.6], where Δτ = τ[3.6] ? τ[13], is about 0.3 over most of ring C, indicating a substantial fraction of centimeter-size particles. Some narrow imbedded ringlets show marked increases in Δτ/τ[3.6] near their edges, implying an enhancement in the relative population of centimeter-size and smaller particles at those locations. In the Cassini division, several sharply defined gaps separate regions of opacity τ ～ 0.08 and τ ～ 0.25; the opacity in the Cassini Division appears to be nearly independent of λ. The boundary features at the outer edges of ring C and the Cassini Division are remarkably similar in width and opacity profile, suggesting a similar dynamical control. Ring A appears to be nearly homogeneous over much of its width with 0.6 < τ[3.6] < 0.8 but with considerable thickening, to τ[3.6] ～ 1.0, near its inner boundary with the Cassini division. Normalized differential opacity decreases from ～0.3 at the inner and outer edges of ring A to Δτ/τ[3.6] ～ 0 at a point about one-third of the distance from the inner edge to the outer. The inner one-fourth of ring B has τ[3.6] ～ 1.0, except very near the boundary with ring C, where it is greater. The outer three-fourths of ring B has $\text{τ[3.6] ? 1.2}$. The differential opacity for the inner one-fourth of ring B is Δτ/τ[3.6] ～ 0.15. There are no gaps in ring B exceeding about 2 km in width. Ring F was observed at 3.6 cm as a single ringlet of radial width $\text{? 2}\text{km}$, but was not detected in 13 cm data.  相似文献   

Atmospheric loss of energetic electrons in the Jovian synchrotron zone     

K. Wang  S.J. BoltonS.M. Gulkis  S.M. Levin 《Planetary and Space Science》2002,50(3):277-285

For decades, ground-based radio observations of Jovian synchrotron radiation have shown emission originating predominantly from the equatorial region and from high-latitude regions (lobes) near L∼2.5. The observations show a longitudinally asymmetric gap between the emission peaks of the lobes and the atmosphere of Jupiter. One possible explanation for these gaps is the loss of electrons through collisions with atmospheric neutrals as the electrons bounce along magnetic field lines and drift longitudinally in the presence of asymmetric magnetic fields. To assess this hypothesis, we applied the recently developed O6 and VIP4 magnetic field models to calculate the trajectories of electrons as they drift longitudinally in Jupiter's magnetic field, and derive the sizes of their equatorial drift loss cones. We then identified the shells on which electrons would be lost due to collisions with the atmosphere. The calculated drift loss cone sizes could be applied in future to the modeling of electron distribution functions in this region and could also be applied to the study of Jovian auroral zone. This method also allowed us to compute the shell-splitting effects for these drifting electrons and we find the shell-splitting to be small (?0.05R_J). This justifies a recent modeling assumption that particles drift on the same shells in a three-dimensional distribution model of electrons. We also compared the computed gaps with the observed gaps, and found that the atmospheric loss mechanism alone is not able to sufficiently explain the observed gap asymmetry.  相似文献   

Effects of ion collisions on the interaction of transverse waves with half-space plasma     

R. K. Singh  D. R. Phalswal 《Astrophysics and Space Science》1985,108(2):409-414

For half-space (Z>0), homogeneous, collisonal and warm plasma, the expressions for fields and penetration depth δ/δ _e (in the unit of ion collisionless cold plasma penetration depth, i.e., when v _i =0, υ_0i =) are derived and discussed numerically. It is concluded that the propagation of transverse waves is only slightly affected by the ion collisions and the applied magnetic field when the plasma frequence is greater than the wave frequency (ω _pe >ω). For the case of ω _pe ≤ω, the damping of the wave is not affected by the changes in the ion collision frequency and the ion temperature. However, in this case, the propagation of the wave is drastically affected by the applied magnetic field and the wave damps quickly as the magnetic field strength or the gyrofrequency (Ω _e ) increases.  相似文献   

Statistical mechanics of Keplerian orbits     

K. A. Hämeen-Anttila 《Astrophysics and Space Science》1975,37(2):309-333

A statistical theory of Keplerian orbits is constructed for a system of particles, which are subject to partially elastic collisions. If the elasticity decreases with collisional velocity, the system shows an increased tendency to form condensations. Near the central body they are concentric rings, which are separated by gaps void of matter. At larger distances outside the Roche limit, the condensations probably form larger bodies. An application to Saturn's rings suggests that at least rings A and C would consist of separate ringlets.  相似文献   

The Christiansen Effect in Saturn’s narrow dusty rings and the spectral identification of clumps in the F ring     

M.M. Hedman  P.D. Nicholson  R.H. Brown  R.N. Clark  C. Sotin 《Icarus》2011,215(2):695-711

Stellar occultations by Saturn’s rings observed with the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft reveal that dusty features such as the F ring and the ringlets in the Encke and the Laplace Gaps have distinctive infrared transmission spectra. These spectra show a narrow optical depth minimum at wavelengths around 2.87 μm. This minimum is likely due to the Christiansen Effect, a reduction in the extinction of small particles when their (complex) refractive index is close to that of the surrounding medium. Simple Mie-scattering models demonstrate that the strength of this opacity dip is sensitive to the size distribution of particles between 1 and 100 μm across. Furthermore, the spatial resolution of the occultation data is sufficient to reveal variations in the transmission spectra within and among these rings. In both the Encke Gap ringlets and F ring, the opacity dip weakens with increasing local optical depth, which is consistent with the larger particles being concentrated near the cores of these rings. The Encke Gap ringlets also show systematically weaker opacity dips than the F ring and Laplace Gap ringlet, implying that the former has a smaller fraction of grains less than ∼30 μm across. However, the strength of the opacity dip varies most dramatically within the F ring; certain compact regions of enhanced optical depth lack an opacity dip and therefore appear to have a greatly reduced fraction of grains in the few-micron size range. Such spectrally-identifiable structures probably represent a subset of the compact optically-thick clumps observed by other Cassini instruments. These variations in the ring’s particle size distribution can provide new insights into the processes of grain aggregation, disruption and transport within dusty rings. For example, the unusual spectral properties of the F-ring clumps could perhaps be ascribed to small grains adhering onto the surface of larger particles in regions of anomalously low velocity dispersion.  相似文献   

A semi-analytic model for oligarchic growth     

John Chambers 《Icarus》2006,180(2):496-513

A new semi-analytic model for the oligarchic growth phase of planetary accretion is developed. The model explicitly calculates damping and excitation of planetesimal eccentricities e and inclinations i due to gas drag and perturbations from embryos. The effects of planetesimal fragmentation, enhanced embryo capture cross sections due to atmospheres, inward planetesimal drift, and embryo-embryo collisions are also incorporated. In the early stages of oligarchic growth, embryos grow rapidly as e and i fall below their equilibrium values. The formation of planetesimal collision fragments also speeds up embryo growth as fragments have low-e, low-i orbits, thereby optimizing gravitational focussing. At later times, the presence of thick atmospheres captured from the nebula aids embryo growth by increasing their capture cross sections. Planetesimal drift due to gas drag can lead to substantial inward transport of solid material. However, inward drift is greatly reduced when embryo atmospheres are present, as the drift timescale is no longer short compared to the accretion timescale. Embryo-embryo collisions increase embryo growth rates by 50% compared to the case where growth is solely due to accretion of planetesimals. Formation of 0.1-Earth-mass protoplanets at 1 AU and 10-Earth-mass cores at 5 AU requires roughly 0.1 and 1 million years respectively, in a nebula where the local solid surface density is 7 g cm⁻² at each of these locations.  相似文献   

The vertical structure and thickness of Saturn's rings     

Jeffrey N. Cuzzi  Richard H. Durisen  Joseph A. Burns  Patrick Hamill 《Icarus》1979,38(1):54-68

We have considered the steady state vertical structure of Saturn's rings with regard to whether collapse to a monolayer due to collisions between particles, the end state predicted by Jeffreys (1947a), may be prevented by any of a variety of mechanisms. Given a broad distribution of particle sizes such as a typical power law n(R) = n₀R^?3, it is found that gravitational scattering of small particles by large particles maintains a true ring thickness of several times the radius of the largest particles, or many times the radius of the smallest particles. Thus the “many-particle-thick” condition which best satisfies optical observations, such as the opposition effect, may be reconciled with ongoing particle collisions. If we consider the obvious sources of energy available for such a process, we find that a ring thickness of only tens of meters may be sustained over the lifetime of the solar system. This implies a maximum particle size on the order of a few meters.  相似文献   

Partially elastic collisions in dense matter     

Liisa Nygrén 《Astrophysics and Space Science》1976,39(2):313-319

The effects of partially elastic collisions on a dense system of particles moving in Keplerian orbits are studied. As in the case of a low-density system (Hämeen-Anttila, 1975), evolution leads to the formation of separate ringlets. The results reveal an anisotropic structure of matter, which may explain some peculiarities in the photometry of Saturn's rings.  相似文献   

Nonlinear effects associated with the finite frequency pump kinetic Alfvén wave and turbulent spectrum in solar wind     

R. P. Sharma  K. V. Modi 《Astrophysics and Space Science》2014,352(1):17-22

The paper contains a numerical simulation of the nonlinear coupling between the kinetic Alfvén wave and the ion acoustic wave for an intermediate β-plasma (m _e/m _i?β?1). For this study, we have introduced the nonlinear ponderomotive force (due to the finite frequency (ω ₀<ω _ci) kinetic Alfvén wave) in the derivation of the ion acoustic wave. The main aim of the present paper is to study the nonlinear effects associated with the different driving finite frequencies (ω ₀<ω _ci) of the pump kinetic Alfvén wave on the formation of localized structures and a turbulent spectrum applicable to the solar wind around 1 AU. As a result, we found that the different driving frequencies of the pump kinetic Alfvén wave affect the formation of the localized structures. We have also studied the turbulent scaling which follows (～k ^?3.6) for ω ₀/ω _ci≈0.2, (～k ^?3.4) for ω ₀/ω _ci≈0.3 and (～k ^?3.2) for ω ₀/ω _ci≈0.4, at small scales. Further, we have also found that different finite driving frequencies of the pump kinetic Alfvén wave affect the turbulence scaling at small scales, which may affect the heating of the plasma particles in solar wind. The present study is correlated with the observation made by the Cluster spacecraft for the solar wind around 1 AU.  相似文献   

N-body simulations of viscous instability of planetary rings     

Heikki Salo  Jürgen Schmidt 《Icarus》2010,206(2):390-409

We study viscous instability of planetary rings in terms of N-body simulations. We show that for rings composed of fairly elastic particles (e.g. as in Hatzes et al. [Hatzes, A., Bridges, F.G., Lin, D.N.C., 1988. Collisional properties of ice spheres at low impact velocities. Mon. Not. R. Astron. Soc. 231, 1091-1115]) the instability may lead to the spontaneous formation of dense ringlets in a background of lower density. In most parts of Saturn’s rings the particle collisions are probably much more dissipative, as suggested by the presence of self-gravity wakes, and classic viscous instability should be suppressed. However, our results demonstrate that the mechanism of viscous instability itself is valid. The dynamical effects of size-dependent elasticity in a system with a size distribution have never been studied before. We show that this may in principle lead to a size-selective viscous instability, small particles concentrating on ringlets against the more uniform background of large particles.  相似文献   

Role of Coulomb collisions in the equatorial F-region plasma instabilities     

A. Bhattacharyya  G.S. Lakhina 《Planetary and Space Science》1984,32(5):635-640

The effect of collisions on electrostatic instabilities driven by gravity and density gradients perpendicular to the ambient magnetic field is studied. Electron collisions tend to stabilize the short wavelength (k_y?_i ? 1, where k_y is the perpendicular wavenumber of the instability and ?_i is the ion Larmor radius) kinetic interchange mode. In the presence of weak ion-ion collisions, this mode gets converted into an unmagnetized ion interchange mode which has maximum growth rate one order smaller than that of the collisionless mode. On the other hand, electron collisions can excite a long wavelength resistive interchange mode in a wide wavenumber regime ($\text{10}{}^{\mathrm{?}}\text{3 ? k}{}_{\mathrm{y}}\text{?}{}_{\mathrm{i}}\text{? 0.3}$) with growth rates comparable to that of the collisional Rayleigh-Taylor mode. The results may be relevant to some of the spread F irregularities.  相似文献   

Optical thickness of the Cassini division     

K.A. Hämeen-Anttila 《Icarus》1977,31(3):385-400

Various estimates for the optical thickness of the Cassini division are studied in order to explain and eliminate the discrepancies between them. An analysis of dark-side observations and a theoretical study based on the behavior of collisions suggest that the optical thickness of the Cassini division is not constant, but fluctuates in the range of 10^?4–10^?3. The nonzero brightness in reflected light is caused either by stray light or by narrow optically thick ringlets inside the Cassini division.  相似文献   

Solar system history as recorded in the saturnian ring structure     

Hannes Alfvén 《Astrophysics and Space Science》1983,97(1):79-94

The paper is based on Holberg's analysis of the Voyager photographs in both reflected and transparent light, combined with occultation data of stars seen through the rings. Besides rapidly varying phenomena (spokes, braided ring, etc.), which according to Mendis are due to gravito-electromagnetic effects, the ring consists of abulk structure, a fine structure, and also ahyperfine structure, showing more than 10000 ringlets. The large number of ringlets can be explained by the Baxter-Thompson ‘negative diffusion’. This gives the ringlets a stability which makes it possible to interprete them as ‘fossils’, which originated at cosmogonic times. It is shown that thebulk structure can be explained by the combined ‘cosmogonic shadows’ of Mimas, the co-orbiting satellites, and the Shepherd satellites. This structure originated at the transition from the plasma phase to the planetesimal phase (which probably took place 4–5×10⁹ y ago). Further, Holberg has discovered that the shadows are not simple void region but exhibit a certain characteristic ‘signature’. This is not yet understood theoretically. Parts of thefine structure are explained by Holberg as resonances with the satellites. Parts are here interpreted as cosmogonic shadow effects. However, there are a number of ringlets which can neither be explained by cosmogonic nor by resonance effects. The most important conclusion is that an analysis of the ring data is likely to lead to areconstruction of the plasma-planetesimal transition with an accuracy of a few percent.  相似文献   

Direct physical collisions and the evolution of dense stellar systems     

Henry E. Kandrup 《Astrophysics and Space Science》1985,109(2):215-230

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Relaxational dissipation in atmospheric waves—I. Basic formulation     

C.O. Mines 《Planetary and Space Science》1977,25(11):1045-1060

Relaxation of molecular thermal energy between translational and internal degrees of freedom, which occurs in the course of molecular collisions, acts to dissipate dynamical energy of the atmosphere and so produces attenuation of atmospheric waves. This paper establishes a basic formalism by means of which the effects may be incorporated readily into dynamical studies. It does so with the aid of a frequency-dependent complex thermal capacitance, c_ω, for which general formulae are developed. These formulae are extended to include the effects of radiative relaxation as well, and the imaginary part of c_ω is recast in a generalized form of Newtonian cooling coefficient; the relative importance of collisional relaxation and radiative relaxation in the dissipation of dynamical energy may be assessed readily with the aid of either parameter.The ‘volume viscosity’ that is sometimes attributed to the atmosphere is shown to have its origins in the same relaxation processes and to be nothing other than an alternative to C_ω, for expressing their effects, an alternative that is inferior and even illegitimate in some circumstances and therefore one that should be abandoned in favour of c_ω.Though the paper is largely tutorial in nature, it establishes generalizations and limitations that appear to be absent from other work. Its formal results are applied to waves in the Earth's upper atmosphere in a companion paper.  相似文献